Electro-acoustic transducers, such as surface acoustic wave (SAW) resonators and bulk acoustic wave (BAW) resonators, are used in a wide variety of electronic applications, such as cellular telephones, smart phones, electronic gaming devices, laptop computers and other portable communications devices. BAW resonators include a resonator stack mechanically isolated from a substrate by an acoustic reflector. For example, BAW resonators include film bulk acoustic resonators (FBARs), which have a resonator stack isolated from a substrate by a cavity that which functions as the acoustic reflector, and solidly mounted resonators (SMRs), which include a resonator stack isolated from a substrate by a stack of alternating layers of low acoustic impedance material and high acoustic impedance material (e.g., an acoustic Bragg reflector). BAW resonators are used to construct electrical filters and voltage transformers, for example.
Typically, a BAW resonator has a layer of piezoelectric material between two electrodes. Examples of the piezoelectric material include aluminum nitride (AlN), zinc oxide (ZnO), and lead zirconate titanate (PZT). Aluminum nitride is advantageous since aluminum nitride maintains its piezoelectric properties at a high temperature (e.g., above 400° C.).
A BAW resonator has a series electrical resonance and a parallel electrical resonance. The series resonance is at a series resonant frequency Fs at which a dipole vibration in the resonator stack is in phase with the electric field applied to the piezoelectric layer. On a Smith Chart, the series resonance frequency Fs is the frequency at which the Q circle crosses the horizontal axis. The series resonance frequency is governed by, inter alia, the total thickness of the resonant stack. Decreasing the total thickness of the resonant stack increases the series resonance frequency. Moreover, the thickness of the piezoelectric layer determines the bandwidth of the BAW resonator. Specifically, a certain electromechanical coupling coefficient kt2 is needed for the series resonance of the BAW resonator to have a specified bandwidth. The kt2 of a BAW resonator is influenced by several factors, such as the respective dimensions (e.g., thickness) of the piezoelectric layer and of the electrodes, and the composition and structural properties of the piezoelectric material and the electrode material(s). Typically, for a particular piezoelectric material, increasing the thickness of the piezoelectric layer increases kt2. Once the bandwidth is specified, kt2 is set, and the thickness of the piezoelectric layer of the BAW resonator is fixed.
Recently, piezoelectric materials having a different relationship between kt2 and the thickness of the piezoelectric layer have been disclosed. These materials enable a defined kt2 to be obtained using a thinner piezoelectric layer and thicker electrodes. Additionally, electrode structures that increase the Q-factors of the resonances (and the Q-factor of the parallel resonance in particular) have been disclosed. BAW resonators incorporating these developments have better performance than conventional BAW resonators. However, in some cases, properties of the improved BAW resonators are more temperature dependent than those of conventional BAW resonators
What is needed, therefore, is an improved BAW resonator having properties that are less temperature dependent.